1. Field of the Invention
The present invention relates to a sheet feeder for supplying sheets for image forming to an image forming section and an image forming apparatus incorporating this sheet feeder. More particularly, it relates to a sheet feeder including a feed roller for feeding stored sheets one by one to the image forming section and a lifting plate for lifting the sheets up to a feeding height (position) at which each sheet is to be fed by the feed roller, the sheet feeder being arranged to raise the lifting plate by pull-up using a wire, and an image forming apparatus incorporating the sheet feeder.
2. Description of Related Art
In a sheet feeder of an image forming apparatus, an uppermost one of sheets stored therein is fed out by a feed roller. The sheets are stacked on a lifting plate and lifted up by the lifting plate. Irrespective of the number of stored sheets, accordingly, the uppermost sheet is placed in contact with the feed roller. In some sheet feeders, the lifting plate is raised, or moved upward, by pull-up using a wire. Specifically, a wire attached to the lifting plate is wound to raise the lifting plate, thereby lifting up the sheets.
Meanwhile, the sheet feeder is sometimes pulled out from the image forming apparatus in order to replenish sheets. When the sheet feeder is pulled out from the image forming apparatus, the lifting plate has to be rapidly lowered, or moved downward, to allow a user to easily replenish sheets. In some cases, some electrical parts or components are arranged under the lifting plate in the sheet feeder. In case the lifting plate remains moved up, a user may carelessly touch the electrical parts. Also to avoid such an accident, the lifting plate needs to be moved downward quickly.
This downward movement of the lifting plate is basically caused by its own weight and the weight of the sheets stacked thereon. In the case where a large amount of sheets are stacked, accordingly, the downward moving speed is apt to be fast and large impact is likely to occur when the lifting plate lands on a lower limit position. To avoid this, a braking member against the downward movement is provided to prevent the downward moving speed from becoming too fast even when a large amount of sheets are stacked. When the number of stacked sheets is close to zero, to the contrary, this braking member alone may make the downward moving speed too slow because excessive braking is caused. JP-A-8(1996)-127434 therefore discloses a technique that a plurality of braking members are provided and the number of jointed braking members is changed by use of partially toothed gears according to circumstances.
However, the technique disclosed in JP-A-8(1996)-127434 requires a complicated structure and thus high cost due to the necessity of providing the plurality of braking members. This technique also needs a special component such as the partially toothed gears.
The present invention has been made to solve the above problems and has a purpose to provide a sheet feeder having a simple structure capable of rapidly moving downward a lifting plate and preventing impact of the same, and an image forming apparatus incorporating the sheet feeder.